1. Field of the Invention
The present invention relates to light valves, and in particular, to a process for forming a light valve pixel cell utilizing a final rapid thermal anneal (RTA) step.
2. Description of Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These display devices typically include a light source which passes light through a light valve.
One of the methods for producing colors in a liquid crystal display is to sequentially project light having a wavelength corresponding to a primary color onto a single light valve. Color sequential light valves create a spectrum of color within the range of the human perception by switching between a set of discrete primary colors. Typically, red, green, and blue are the primary tri-stimulus colors used to create the remaining colors of the spectrum.
Specifically, during projection of each primary color, the light intensity is modulated such that combination of the intensities of the primary colors in sequence produces the desired color. The frequency of switching between the primary wavelengths by the light valve should be sufficiently rapid to render discrete primary states indistinguishable to the human eye.
Two factors dictate the minimum frequency necessary for switching. The first factor is the ability of the human eye to detect the discrete primary colors (e.g., red, green, blue). At slower than ideal switching speeds, the human eye will detect a flicker and the primaries may not blend.
The second factor determining the frequency of switching is the video refresh rate. During display of video images, the individual frames must be refreshed at frequencies undetectable to the human eye.
The net frequency of switching demanded by the combination of sequential color blending and video refreshing is beyond the capabilities of light valves that utilize thick (&gt;1.mu.m) liquid crystal (LC) transducers. However, thin (&lt;1.mu.m) liquid crystal transducers have been successfully fabricated. These thin LC transducers demonstrate adequate color sequential blending at video refresh rates. One example of such a thin LC transducer pixel cell structure is disclosed in U.S. Pat. No. 5,706,067, to Colgan et al.
In general, the conventional thin LC transducer pixel cells possess enhanced responsiveness due to the decreased volume of liquid crystal material between the top and bottom plates. A smaller volume enables the liquid crystal to shift orientation more quickly and in response to a lower applied voltage.
FIG. 1A shows a plan view of adjacent thin LC transducer pixel cells in a conventional light valve. FIG. 1B shows a cross-sectional view of the adjacent pixel cells of FIG. 1A across line 1B-1B'. Light valve portion 100 comprises adjacent pixel cells 110a and 110b having liquid crystal (LC) material 111 sandwiched within gap 106 between a top plate and a bottom plate. The top plate is composed of a translucent material, typically glass. The underside of the top plate is coated with optically transparent and electrically conducting material, typically indium-tin oxide (ITO). This conductive layer serves as a passive electrode for the active pixels below. This passive electrode layer also typically bears a polyimide layer, which is scored to provide an anchoring alignment for the LC material 111.
The bottom plate of the pixel cell is formed by the active reflective metal pixel electrodes 112a and 112b of adjacent pixels 110a and 110b, respectively. Pixel electrodes 112a and 112b are separated and electrically isolated by trench 118.
Trench 118 is filled with dielectric material 121. Dielectric material 121 also extends over the surface of the active pixel electrodes, performing a passivation, planarization, and/or LC alignment function.
Pixel electrodes 112a and 112b lie on top of an upper intermetal dielectric layer 128 that forms a component of interconnect scheme 104. Interconnect 104 overlies capacitor structures 120a and 120b formed within underlying silicon substrate 105.
Storage capacitors 120a and 120b are in electrical communication with pixel electrodes 112a and 112b, respectively, through metal-filled vias 140, middle interconnect metallization layer 124, and lower interconnect metallization layer 122. Storage capacitors 120a and 120b are controlled by MOS switching transistors 142a and 142b, respectively. MOS switching transistors 142a and 142b are also formed in underlying silicon substrate 105, and are electrically isolated from adjacent semiconducting devices by trench isolation structures 144.
The conventional pixel cell described above in FIGS. 1A-1B functions adequately in many applications. However, this design suffers from a number of disadvantages.
One problem is that favorable optical characteristics of the pixel cell can be diminished by specific processing steps utilized during its manufacture. In particular, one important measure of pixel cell performance is reflectance, which determines both brightness and resolution of the image presented. Reflectance of the pixel cell is determined by reflectance of the active pixel electrode, and the reflectance of this electrode can be adversely affected by heat during fabrication of the device.
Therefore, there is a need in the art for a process for fabricating a pixel cell which preserves reflectance of the active pixel electrode.